The present invention relates to a headbox for a machine for the manufacture of fibrous webs made from stock suspensions, and more particularly for the manufacture of paper webs. The headbox is of the type having an outlet duct. The duct has lateral side walls which define a determined duct width The duct is further bordered by two duct walls, respectively located above and below, and converging in the direction of flow of suspension to form an outlet slot in the downstream region. One duct wall is movably and adjustably supported by a duct wall beam, and is preferably pivotally supported at its upstream end, so that the clear width of the outlet slot can be varied by means of a lifting appliance. The bearing force of that appliance acts against the pressure of the suspension which acts on the movable duct wall. A support beam extends across the width of the machine adjacent to the adjustable duct wall beam. The movable duct wall beam together with the support beam extends over the machine width and forms a beam unit. A pressure cushion, which acts against the suspension pressure that acts on the movable duct wall, is disposed between the movable duct wall beam and the support beam.
A headbox of this type is known from German Published Application 3,614,302, which corresponds to U.S Pat. No. 4,770,745. In this publication, the movable duct wall is part of a U-shaped, L-shaped or box-shaped duct wall beam. The support beam, which is different from the duct wall beam, may have an I-shaped or a triangular shaped cross section. In all cases, there is a pivotable connection between the duct wall beam and the support beam located at both ends of the duct wall beam This is known from an advertisement entitled, "Unsere Stufendiffusor-Stoff-Auflaufe sind Marketfuhrer" of Sulzer Escher Wyss, published in the November 1987 issue of "Wochenblatt for Papierfabrikation".
This German application also discloses that the structural unit, which consists of the duct wall beam and the support beam and is hereinafter known as the "beam unit", is connected to the headbox housing by means of a pivot bearing. At each end of the beam unit there is also a lifting appliance, e.g. a spindle, by means of which the movable duct wall can be swung up or down, to vary the clear width of the outlet slot. For this purpose, the movable duct wall is connected to the headbox housing by means of a hinge joint. The publication provides no guidance about how the lifting appliance is hingedly connected to the beam unit However, it is customary to provide a journal hinge at each end of the duct wall beam. See FIG. 4 herein.
A major problem with such headboxes is keeping the clear width, i.e. the space between the top and bottom duct walls, of the outlet slot constant with the greatest possible accuracy over the entire machine width. In practice, it has repeatedly been shown that local deviations in the desired outlet slot width impair the quality of the resulting paper web. Investigations have shown that a W-shaped or M-shaped profile of the cross section of the outlet slot is usually found, which produces a corresponding irregular "weight per unit area transverse cross section" (usually called "cross direction profile of the basis weight") of the paper web. It has also been shown that a certain error in the slot width of the outlet slot can cause a ten fold error in the basis weight of the paper web.
It has also been recognized that some of the problems were caused by the above mentioned articulation of the two lifting appliances located at the two ends of the duct wall beam, i.e. on the front side and the drive side of the paper making machine, by means of a journal hinge Previously, at each end of the movable duct wall itself, a journal extended at right angles to the machine direction and was hinged to the lifting appliance. A large portion of the forces originating from the pressure of the suspension from the movable duct wall must be transferred by the respective journal located on the movable duct wall to the stationary headbox housing. As a result, the journal transmits transverse force and a bending moment into the movable duct wall This has disadvantageous effects on the contours of the movable duct wall for the following reason. The loading of the movable duct wall which originates from the stock suspension is distributed substantially uniformly over the length of the wall, i.e. over the machine width. However, from the opposite direction, the loading of the movable duct wall is composed of a uniformly distributed load produced by the compressed air cushion and by the transverse forces and the bending moments produced by the lifting device. It has been shown that satisfactory results were not achieved under these conditions.
The same comment also applies for the headbox disclosed in Voith (the assignee hereof) publication p. 2503, page 4. For the reasons stated, repeated attempts have been made to improve the uniformity of the outlet slot width by additional measures. More particularly, at the outlet slot, a locally deformable component, e.g. a profile bar, was provided. This could be adjusted by means of a plurality of spindles uniformly distributed over the machine width However, the accuracy which can be achieved by this measure is frequently not sufficient to meet current requirements for paper quality.